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Starter we designed that used that type
of power supply, and it explained how
it worked.
Fundamentally, they are pretty simple. At a fixed frequency (50Hz for
our mains supply), the impedance of
a capacitor is inversely proportional
to its value.
So with a capacitor connected
in series with the supply, the current drawn depends on the capacitor value; double the value, and you
roughly double the current that the
device draws.
The capacitor applies the mains
voltage waveform to a bridge rectifier,
with its output voltage being clamped
by a zener diode or similar to produce
a DC rail. Since the capacitor limits
the current being fed in, the zener is
not destroyed and the desired supply
voltage is achieved.
So the capacitor value is chosen
based on how much current the circuit will consume; making it larger
than necessary will just waste power,
and if it's too small, the desired supply voltage will not be achieved. So
any significant drop in capacitance is
likely to stop the device from working. The only solution is to replace
that capacitor.
Unfortunately, it is not uncommon
for X2 capacitors to lose capacitance
over time. Money and space pressure
sometimes cause manufacturers to
choose capacitors that are physically
too small to be reliable. Also, by the
nature of the way X2 safety capacitors are designed, being exposed to
voltage spikes (eg, caused by nearby
lightning) will often cause them to lose
capacitance.
Question about
an old project
I can’t find anything on your website
about the PCB used in the September
1992 LCD Readout. I have the used
PCB here for testing; the code on it is
00921. I’d love to make a spare as this
unit has become well worn. Mostly I
use it for testing displays before fitting
them to other equipment. Is this PCB
pattern available?
I have been subscribed since the
early 1980s. I may be able to find the
magazine in my attic storage, it would
take some time, and it’s hot up there.
(J. P., Shailer Park, Qld)
● 00921 does not look like a Silicon
Chip PCB code as our codes are usually eight digits long.
We can’t find any projects in September 1992 called “LCD Readout”,
but there is a General-Purpose 3 1/2Digit Panel Meter project (siliconchip.com.au/Article/5520) that uses
an LCD screen. Presumably, that is
the one you are referring to. Its PCB
code is 04110921, similar to the code
you provided.
We don’t sell many PCBs from
that long ago (as there is virtually no
demand), nor do we have any CAD or
other computer files for that designs
before about 1995. The pattern was
published in the article on page 85. It
looks like it was made with tape, not
on a computer, which is likely why we
don’t have any files from back then.
We have scanned the PCB pattern,
cleaned it up, and added the image
to our website’s PCB Patterns download section (siliconchip.com.au/
Shop/10/6329).
SC
Notes & Errata
Dual Hybrid Power Supply part two, March 2022: in Fig.11 on page 85, the metal sheet for the heatsink folds up where shown,
not down. Also, the hole in the heatsink should be drilled 25mm from the left edge, not 30mm. Note that link LK1 on the control
board, shown in Fig.13 on page 86, needs the shorting block in the upper RDO position, not the lower SDO4 position. Finally,
instead of the 15μF tantalum capacitors specified, non-polarised 15μF 50V X7R M5750/2220 ceramics can be used. We supply
those in the kits as they are superior to the tantalum caps in virtually every way.
Remote Control Range Extender, January 2022: if needed, the optional pull-down resistor at pin 1 of the IC1 (PIC10LF322) on
the transmitter should be 1kW rather than 100kW as originally specified. This lower value ensures the resistor is reliably detected
and the internal pull-up is always disabled.
Programmable Hybrid Lab Supply with WiFi, May & June 2021: on p74 of the June issue, Fig.7 shows the copper layers swapped
and thus the SMD components are shown placed on the wrong side of the board and mirrored. The actual locations of some of
these parts can be seen in the photo at the bottom of p75 of the same issue, and the diagram has been corrected in the June
2021 online issue. This means that two 1.8kW resistors are under the ESP-32 module.
The June 2022 issue is due on sale in newsagents by Thursday 26th of May. Expect postal delivery of subscription copies
in Australia between May 24th and June 10th.
112
Silicon Chip
Australia's electronics magazine
siliconchip.com.au
